One form of multiprocessor computer architecture consists of stacked multiple printed circuit boards, each board containing several computing nodes. When a computing node includes multiple chips for performing processing, memory, communications and other functions as needed, the node may be referred to as a "multi-chip module," or "MCM."
Typically, although not necessarily, an MCM is formed with a surface geometry of a rectangle or, more usually, a square. The choice of a square surface geometry is influenced by conventional design, fabrication and MCM interconnection technology. In this typical configuration, each of the four edges of each MCM serve as a port, where a multiplicity of signal wires to and from the MCM are routed. The ports typically are denoted north, east, west and south. The ports physically are a linear array of contact pads disposed along the MCM edge, to a density typically of 125 pads per running inch.
One way to realize useful arrays of MCMs is to mount, for example, four square-shaped MCMs on an individual 4-sided metal core board. In addition to providing a mounting substrate for the MCMs, the metal core board provides linear fields of contact pads corresponding to the contact pads along the four sides of the MCM, and electrical connections, selectively as needed, between the MCM contact pads and the board contact pads. The latter contact pads provide the means for connecting the MCM ports on each board to the "outside world," including the MCMs located on boards at other layers in the stack, and a Host computer.
A preferred approach for stacking and electrically connecting the boards of a stacked array is described, for example, in the earlier patent application of the present Applicants, Ser. No. 07/590246, filed Sep. 28, 1990, and assigned to Applicant's assignees.
The complexities of the computer architecture formed by the MCMs in the stacked board arrays require a very large number of necessary signal paths. These signal paths are formed and routed on the MCMs themselves, as well as between MCMs on the same board level and MCMs which are located on boards disposed at other levels in the stack.
Large numbers of signal paths greatly increase the incidences of path "crossover" points. Crossover points occur both within the MCM and on the metal core board. Since the paths at cross-over points must not electrically contact one another, board and MCM design strategies must be used to prevent interfering electrical contact between crossing paths.
The typical expedient is to re-route one of the paths to a different layer and thus "cross over" the first path in a different plane. This approach requires "vias," or vertical interconnections fashioned as plated-through holes; lased vias (sometimes termed "microwire") or microvias. These interconnections join the upper and the lower level of the circuit, wherein one of the interfering circuit paths is connected to the top end of the via. The path is continued through to the bottom end of the via, where a connection is made to a continuation path formed on the underside of the device, substrate or the board, as the case may be.
Vias on the mounting board, however, detract from the very limited surface area needed for running circuit paths. Vias also add more detail to the board. Thus, an excess of vias creates increased board size and/or number of layers; and adds substantially to production costs. When vias are implemented as plated-through holes, they increase total area requirements because of the added land area and clearance area required; and also can decrease board yield. The vias here also reduce the heat dissipation of the board by detracting from the metalization layer.
The efficient routing of signals in the stacked board configuration such as is described in the noted patent application, therefore has been difficult to achieve because of the lack of a way to minimize the number of vias on the metal core board.